1. Field of the Invention
The present invention relates generally to a switch mechanism for a key-entry use and, more particularly, to a key-entry switch (hereinafter referred to as a key switch) preferably used for a relatively thin keyboard incorporated in a portable electronic equipment, such as a notebook-size personal computer or word processor. The present invention also relates to a relatively thin keyboard provided with a plurality of key switches having such structures.
2. Description of the Related Art
In the technical field of portable electronic equipment, such as notebook-size personal computers or word processors, etc., various techniques have been provided, which can facilitate the reduction of height or thickness of an equipment housing including a keyboard, to improve the portability of the equipment. Particularly, when the height of a keyboard provided with a plurality of key switches is reduced, it has been generally required to maintain the stroke of each key switch at a predetermined distance to ensure a constant operational properties thereof and, simultaneously, to reduce the entire height of the key switch upon both the non-operated (or switched-off) and operated (or pushed down and switched-on) conditions thereof.
Japanese Unexamined Utility Model Publication (Kokai) No. 5-66832 (JP-U-5-66832) discloses one example of a key switch for use in such a relatively thin keyboard, which includes a key top adapted to be keyed or pushed down by an operator's finger, a base disposed beneath the key top, a pair of link members for supporting the key top above the major surface of the base and directing it in the vertical or up-and-down direction, a sheet-like switch arranged beneath the base, and an elastic actuating member located between the key top and the sheet-like switch so as to open and close the sheet-like switch corresponding to the vertical or up-and-down movement of the key top.
The pair of link members are pivotably connected with each other, so as to be provided with a generally X-shape in a side view. A first link member is engaged slidably at one end thereof with the base and rotatably at the other end with the key top. A second link member is engaged rotatably at one end thereof with the base and slidably at the other end with the key top. In this manner, the key top is subjected to a parallel displacement in a substantially vertical direction in relation to the major surface of the base, while keeping a predetermined posture of the key top.
The elastic actuating member is a dome-like member integrally formed from a rubber material. The elastic actuating member is placed on the sheet-like switch through an opening formed in the base at a position beneath the key top, with the upper end of the dome facing toward the key top. The sheet-like switch is structured as a pair of conductive contacts opposed to each other and respectively carried on two film-like printed circuit boards. The sheet-like switch is positioned beneath the elastic actuating member normally in an opened state. In this specification, such a contact pair is referred to as a membrane switch, and a pair of film-like boards provided with a membrane switch is referred to as a membrane sheet.
When no external force is applied to the key top, the elastic actuating member supports the key top on the outer surface of the dome upper end, and urges the key top toward an initial position vertically upwardly away from the base. When the key top is pushed downward by a key-entry operation, the elastic actuating member is elastically deformed while exerting a biasing or an elastic restoring force to the key top in an opposite or upward direction. In this condition, a projection formed on the interior surface of the dome upper end serves to push the outer surface of the membrane sheet, so as to close or turn-on the membrane switch. When the downward pushing force applied to the key top is released, the elastic actuating member is elastically restored, so as to return the key top to the initial position and to open or turn-off the membrane switch.
In the above-mentioned conventional key switch, including a pair of link members used as means for supporting/directing the key top, it is possible to fold the link members and put them within a space between the key top and the base as the key top is downwardly displaced. Accordingly, in comparison with other conventional structures including, as means for supporting/directing the key top, a telescopic shaft assembly using a shaft and a bearing which can be slidingly moved relative to each other in a vertical or going up and down direction of the key top, it is possible to further reduce the entire height of the key switch upon both the inoperated and operated conditions thereof, while maintaining the stroke of the key switch at a predetermined distance.
Japanese Unexamined Patent Publication (Kokai) No. 9-27235 (JP-A-9-27235) discloses another example of a key switch also including a pair of link members used as means for supporting/directing a key top. In this key switch, the link members are assembled into a generally X-shape in a side view and are slidably connected with each other at an intersection thereof. Both link members are engaged slidably at one ends thereof with the base and rotatably at the other ends with the key top. In this structure, the key top is also permitted to be subjected to a parallel displacement in a substantially vertical direction in relation to the major surface of the base, and it is also possible to reduce the entire height of the key switch upon both the non-operated and operated conditions thereof, while maintaining a predetermined distance of the stroke of the key switch.
Japanese Unexamined Patent Publication (Kokai) No. 9-190735 (JP-A-9-190735) discloses a further example of a key switch also including a pair of link members used as means for supporting/directing a key top. In this key switch, the link members are assembled into a generally reverse V-shape in a side view and meshed with each other at the toothed ends thereof. Both link members are engaged slidably at one free ends thereof with the base and rotatably at the other toothed ends with the key top. In this structure, the key top is also permitted to be subjected to a parallel displacement in a substantially vertical direction in relation to the major surface of the base, and it is also possible to reduce the entire height of the key switch upon both the inoperated and operated conditions thereof, while maintaining a predetermined distance of the stroke of the key switch.
In a relatively thin keyboard provided with a plurality of key switches each having the above-mentioned pair of link members, a structure is known in which the key top of each key switch is held in an initial projecting position for a key-entry operation during the operating state of the keyboard, while the key top is positively displaced to a retracted position lower than the initial position during the inoperating (or carrying) state of the keyboard, in order to improve the portability of the keyboard.
For example, Japanese Unexamined Patent Publication (Kokai) No. 9-63402 (JP-A-9-63402) discloses a yet further example of a key switch including a dome-shaped elastic actuating member fixedly mounted on a membrane sheet. The elastic actuating member of this key switch can be shifted in a lateral direction together with the membrane sheet in an integral manner under the key top. In the operating state of the keyboard, the elastic actuating member of each key switch is located at a position for supporting the key top thereof in an initial projecting position. On the other hand, during the non-operating state of the keyboard, the elastic actuating member of each key switch is laterally shifted and located at a position where the key top thereof is not supported on the actuating member, and thereby the key top is displaced into a retracted position which corresponds to a pushed-down position in the key-entry operation.
As disclosed in each of the above prior-art documents, the conventional key switch generally utilizes a dome-shaped elastic actuating member as means for opening/closing a membrane switch. The elastic actuating member also serves as means for elastically upwardly biasing the key top away from the base. Therefore, when the actuating member is elastically deformed by a key-entry operation of the key top, the actuating member exerts biasing or elastic restoring force to the key top, which assumes non-linear relationship with a displacement of the key top, due to the dome-shaped profile of the actuating member.
That is, the key switch can establish such a key-entry operating properties that, at the instant when the pushed-down displacement of the key top exceeds a predetermined value, the biasing force, which has been gradually increased until that time, is sharply reduced. As a result, an operator can recognize that the key switch has been correctly and appropriately operated by the finger, even when the keyboard is one adapted to be incorporated in a portable electronic equipment, in which the key switch generally has a relatively short keying stroke.
The dome-shaped elastic actuating member is, however, kept in a location between the key top and the membrane sheet and interposed therebetween in relation to the height of the key switch, regardless of the degree of the deformation of the actuating member. Therefore, the dome-shaped elastic actuating member in itself affects the entire height of the key switch upon both the inoperated and operated conditions thereof. Accordingly, even if the use of the link members as means for supporting/directing the key top could reduce the entire height of the key switch, there is a difficulty in reducing the height or thickness of the key switch and thus of the keyboard, due to the provision of the dome-shaped elastic actuating member.
Further, in the conventional key switch wherein the key top can be displaced into the retracted position by shifting the elastic actuating member in a lateral direction during the inoperating state of the keyboard, it is necessary to define a sufficient space to accommodate not only the link members but also the dome-shaped elastic actuating member not deformed, between the retracted key top and the base. Consequently, the dimension of the key top tends to be increased particularly in the height direction.